Adenotonsillectomy (AT) remains one of the most common surgical procedures performed in children, but indications for AT have changed in recent years. Surgeons now perform AT in many instances for suspected obstructive sleep-disordered breathing (SDB), and sometimes for daytime behaviors that may be a consequence of SDB, especially inattention and hyperactivity. However, whether SDB causes these and other disruptive behaviors, the precise nature of these behaviors, and what types or levels of SDB may be of concern are not well known. Without such knowledge, pediatricians and otolaryngologists do not often make use of objective preoperative testing that could help to assess for SDB or abnormal behavior. The main goals of the research described in this proposal are to (1) better define whether inattention and hyperactivity are frequent among children who undergo AT, (2) identify measures and levels of SDB that are indicative of these behaviors, (3) test whether improvement in SDB after AT is associated with improvement in behavior, and (4) investigate the hypothesis that SDB is a cause of inattention, hyperactivity, and related behaviors in some children. Subjects will be 5 through 12 year-old children recruited after they have been scheduled by their physicians for AT (n = 200) or hernia repair (n = 75 controls). A battery of well-validated behavioral assessment tools, cognitive tests, and structured psychiatric interviews will be used before surgery to define what behaviors are more prominent in the children scheduled for AT rather than hernia repair. All children will undergo preoperative polysomnography which will include, for the first time in such a series, equipment that can detect subtle forms of SDB which may be particularly prevalent in children. Results will allow determination of what polysomnographic findings are associated with well-defined adverse behavioral outcomes. Finally, preoperative and postoperative testing in these subjects will provide a controlled non-randomized trial of AT for SDB, demonstrate whether SDB-associated abnormal behaviors improve after AT, and provide strong evidence for whether SDB is a cause of these behaviors.